Hey i'm currently a student at Case so I can tell you what my experience was like last year.
At the interview you need the basics. Why AA? why not medical school, if you're switching careers be prepared to explain your switch. Why Case Western (especially if you're from another area that has a program) What will you do if you don't get in. What are your strengths/weaknesses. There are not really any unexpected questions, I made sure to have ways to explain anything in my application that looked question worthy. They ask questions like what led you along the path to AA? how did you hear about Case etc. Have questions prepared for the interviewers even if you already know the answers. I did not attend the dinner, but from the other people I interviewed with who did it sounded like an informal meeting with 1st and 2nd year students and a good forum to get questions answered.
This will probably sound cliche but just be yourself. If you got invited for an interview you are academically qualified to be in the program. The interview process is a chance for Case to decide if you are a good fit for the program and for YOU to make sure that it is a good fit for you...
Good Luck!